Electronic counter measures in radar

ABSTRACT

Apparatus and method for eliminating an interference signal from a desired signal where a difference of polarization exists or can be made to exist. The desired signal is received on a first antenna of appropriate polarization and the interference signal is received via another port of the same antenna or a second antenna polarized ninety degrees away from the first. The signal received via the other polarization is adjusted for amplitude and phase so that all interference will be cancelled when both signals are subtracted in a summing junction. In the case where the interference and desired signals do not have polarization differences, provision is made to change the polarization of the desired signal.

PRIOR ART STATEMENT

The three patents described below relate to the interferencecancellation techniques described in this application.

U.S. Pat. No. 3,699,444, INTERFERENCE CANCELLATION SYSTEM, describes aradar system circuit which uses a portion of the transmitted signal,after phase shifting and attenuation, to cancel the transmitted signalreceived at the receiver antenna.

U.S. Pat. No. 3,716,863, INSTRUMENT LANDING ERROR CORRECTING SYSTEMdescribed apparatus for cancelling an interfering signal coherent withthe desired signal, but varying in amplitude and phase.

U.S. Pat. No. 4,016,516, REFLECTIVE SIGNAL CONTROLLER, describes acircuit for varying the amplitude and polarity of an rf signal. Parts ofsaid circuit may be of use in this inventive application.

None of the above references teaches or suggests a technique forcancelling interference based on a difference of polarization between itand the desired signal.

BACKGROUND OF THE INVENTION

Described herein is apparatus for preventing the jamming of radar by anelectronic countermeasure (ECM) jamming or interference signal; and moreparticularly, apparatus for receiving two or more differentpolarizations of the radar return signal and the interference signal,and using that different polarization received signal to cancel theinterference signal without also cancelling the desired radar returnsignal.

It is common for radar systems to be jammed by hostile electroniccountermeasure interference signal sources. These electroniccountermeasure systems typically monitor the radar band, accuratelydetermine the frequency, pulse repetition rate another radar systemcharacteristics, and transmit interfering signals of sufficient powerand of appropriate timing to render the radar inoperative. Thesecountermeasure systems operate in real time so that a change of radarfrequency will instantaneously be followed by a change of interferencefrequency. Therefore, some method of interference avoidance is requiredwhat will act to cancel interference, both pulsed and continuous, evenat the exact frequency of the radar system.

The prior art includes U.S. Pat. No. 3,716,863, INSTRUMENT LANDING ERRORCORRECTION SYSTEM, commonly assigned, which cancels an interferencesignal by producing a correction signal of equal frequency and amplitudebut of appropriate polarity. When the interference, correction anddesired signals are received and summed, the interference signal iscancelled and the desired signal remains. This system is useful wherethe desired and interference signals are of the same frequency and arecoherent but differ in phase. An example is an aircraft receiving an ILSsignal directly from a transmitter and simultaneously receiving areflected ILS signal from a nearby structure.

A system employing this principle may employ a circuit for varying thebasic signal phase and amplitude to produce a correction signal. Such acircuit is described in U.S. Pat. No. 4,016,516, REFLECTIVE SIGNALCONTROLLER, commonly assigned. This signal controller is designed to beinserted into the path between a source and the utilization device toallow the control of signal amplitude ratio and polarity.

The prior art thus recognizes the problem of interfering signals of thesame frequency, and describes the generation of a correction signal ofappropriate phase to cancel said interference. This is possible sincethe correction signal generator is coupled to, and therefore is coherentwith the interference signal source.

In the case where the interference signal is produced by an electroniccounter measure source (ECM), however, there can be no coupling to thesource to generate a coherent correction signal, and cancellation of ajamming signal is not feasible by this method.

An alternative is to cancel such interference by taking advantage of thedifference in signal polarization between desired and interferencesignals. This technique uses circuits equivalent to those required byprior cancellation systems, but requires a different antennainstallation. In the case where the interference and desired signals donot have polarization differences, provision is made to change thepolarization of the desired signal.

SUMMARY OF THE INVENTION

The proposed system comprises a receiver equipped with two receivingantenna ports, each configured to receive waves at a particularpolarization, the two polarizations being ninety degrees out of phasewith each other. For example, one could be configured to receivevertically (V) polarized waves and the other, horizontal (H). Then, tothe extent that the ratios in the interference and radar return signalsare different at the V and H receive ports, a cancellation signal can beobtained by adjusting the amplitude ratio and electrical phase angle ofthe V or H receive port signal that does not normally receive thedesired signal. Therefore, cancellation of the interference signals onlycan be provided even if the desired and interference signals are at thesame frequency.

This system can function wherever the desired versus interference ratiosare not the same at the two polarization ports. This would be true ifthe interference were vertically polarized and the desired signalshorizontally, for instance. Also, the interference may be circularlypolarized and the desired signal, vertically. In the latter case, thehorizontally polarized antenna would receive the interference while thevertically polarized antenna would receive both interference and desiredsignals. A signal received via one channel (port) could be used on theother channel for interference cancellation.

In the case where the interference signals polarization angle is matchedto that of a desired radar signal, the radar polarization angle may bechanged by the operator to avoid cancellation of the desired radarsignals. In the case where the energy is radiated from a radar antenna,the polarization of the radiated signal could be rotated through the useof the appropriate waveguide “plumbing”. The system will then againoperate as stated above.

One variation that the system has to compensate for is the ratio ofreceived interference signal power received by each antenna port. Forinstance, circularly polarized interference has equal amounts ofhorizontally and vertically polarized signal power. Therefore, the sameamount of interference power received at one antenna port has to besubtracted from the signal received at the other antenna port. On theother hand, horizontally polarized interference would provide arelatively small amount of interference at the vertical antenna port.This system variable is the amplitude ratio between the channels. Thesystem automatically reduces (or increases) the amplitude ratio requiredto provide interference cancellation.

Another system variable is the amount of time or phase delay betweenchannels. The same interference signal may be received by one receivingantenna port a fraction of a wavelength ahead of the other. Tocompensate, an electrical phase angle and/or a variable delay in thesystem is provided, so that cancellation signals are produced with theproper electrical phase angle to cancel the interference optimally.

Both the phase and amplitude ratio control circuits are monitored and,to the extent that perfect cancellation was not produced, error signalsare generated. These error signals are then fed back to the phase andamplitude control circuits, closing the loops to make automatic systemcorrections.

Therefore, an object of this invention is to protect a radar receiverfrom interference by differentiating between normal returns andinterference, based on differences between their polarizations and tocancel the interference thus detected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a portion of a receiver systemincorporating this interference cancellation circuit;

FIG. 2 is a detailed diagram of this system, where the amplitude ratioand phase corrections are processed at the intermediate frequency stage;and

FIG. 3 is a schematic diagram of one signal processor embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 contains the main components of the interference cancellationsystem, including vertically polarized 10 and horizontally polarized 11antennas or antenna ports. The invention utilizes a discriminationconcept for the desired and interference signals based on theirpolarization and is, therefore, not constrained by the directions ofarrival of the desired signal and the interference. Consequently, thediscrimination means encompassed by the invention accomodates friendlyand hostile interference arriving through both mainlobes and sidelobes.

Another aspect of this invention based in its polarizationdiscrimination capabilities is that the adaptive nature of thecounter-countermeasure provision makes the interference suppression inthe receiver useful even for moving receivers and interference sources.This is true for two reasons. First, the ratio of received interferencepower between antennas and the interference delay between antennas willnot vary rapidly. Moreover, to the extent that there may be a variation,the closed loop error detecting circuits will automatically compensatefor the variation.

If the interference source is a circularly polarized signal in the samefrequency band as the receiver, and if the desired received signal isvertically polarized; there will be a strong horizontal component of theinterference while the horizontal component of the desired signal willbe negligible. Even if there is de-polarization in the medium, therelative magnitude of the ratio of the desired signal appearing as ahorizontally polarized field component will be significantly lower thanthe ratio of the horizontal and vertical fields of a circularlypolarized jamming signal or interference. In the invention, means areprovided to receive the orthoginal (horizontal) component of theincoming field in addition to the normal (vertical) component such thatthe antenna aperture available to both vertical and horizontal ports isthe same. In other words, the effective receive antenna gains for thevertical and horizontal polarizations are the same or nearly the same.

The signal or interference as received by the horizontal port 10 of theantenna feed is the source for the synthesis of the interference as itappears at the vertical port 11 of the receive antenna. This is due tothe fact that the horizontal and vertical components of the jammingsignal are correlated, except perhaps by an amplitude ratio factor and aphase or time delay. The amplitude ratio factor could be due to thedifferential proportion characteristics of the interference through themedium and the relative antenna orientations. The delay between thevertical and horizontal component of the interference appearing at theradar receive antenna could also be due to similar reasons.

It is evident, therefore, that if an appropriate amplitude ratio factorand a phase or time delay are introduced as transfer functions of thesignal controller 12, as shown in FIG. 1, the output of the signalcontroller will be a cancellation signal which is identically the sameas the interference appearing at the vertical port of the receiveantenna. The subtraction of these two signals, shown in FIG. 1, thenwill yield an output where the interference is cancelled but the desiredsignal is not, since the cancellation of the desired signal cannot occurdue to its “negligible” level at the output of the signal controller. Ifthe synthesized transfer function characteristic of the signalcontroller, that is the amplitude ratio factor K and time (or phase)delay T, are not exactly what are required, the difference between thesynthesized cancelling signal and the interference will not be zero.This nonzero residual interference signal then can be used as the errorsignal of a high-gain servo-loop that drives the factors K and T untilthe error signal vanishes. The equilibrium condition for the loop thenassures the absence of the interference at the receive line.

These functions are accomplished by the apparatus of FIG. 1 as follows:Both horizontally and vertically polarized antenna ports 10 and 11receive rf energy. One antenna port (the horizontally polarized antennain FIG. 1) supplies its output to an amplifier 14 and signal controller12, the latter being used to generate the appropriate power and delayparameters described above.

A signal controller capable of receiving an rf signal, or eitherattenuating or amplifying it, and of varying the amount of delay betweenthe controller input and output are old in the art. See, for example,U.S. Pat. No. 4,016,516 commonly assigned, for a detailed description ofthe signal controller for this application, and which is incorporated byreference herein.

The signal controller output is adjusted for amplitude ratio and delay.This is then subtracted from the vertical port signals in the summingjunction 13.

The output of the summing junction 13 is the correct signal with theinterference deleted. For example, if the interference was receivedmainly at the horizontal antenna, and the desired signal at thevertical; the output of the signal controller would be adjusted tooutput the proper amplitude and phase to cancel the interference in thevertical component.

An error sampling determination is made at a sampler at the output ofthe summing junction 13. Uncompensated errors in the form of rf signallevels are used as inputs to a signal processor 16. These signals areanalyzed for their amplitude and delay relationship to the receivedinterference signals, and correction signals would be applied to thesignal controller 12 to more accurately cancel the interference, thusclosing the error detection loops. An example of a circuit used forcancelling interference not identical to the desired signal in amplitudeand phase is described in U.S. Pat. No. 3,716,863, which is incorporatedby reference herein.

FIG. 2 shows an actual implementation plan for a vertically polarizedradar. In this figure, the signal processing part of the servoloop,referred to above, is effective at some “intermediate frequency” insteadof at the radar receive frequency.

Two probes are mounted in the antenna, one with horizontal polarizationand the other, vertical. The horizontally polarized component 20 iscoupled through an rf limiter 22 and amplifier 23 to a signal controller24 which varies its amplitude ratio and phase angle (or time delay). Theadjusted output is then injected into the vertically polarized channelthrough an rf amplifier 25 and coupler 26 to cancel out the interferencesignal received. The resultant signal is transmitted to the receiverwhere it is used as a regular received signal.

The error signal feedback loop in this embodyment comprises theremainder of the circuit. The orthogonally polarized component is summedthrough coupler 34 with the local oscillator 27 output at junction 30and the difference resulting is amplified in an IF amplifier 28.Similarly, a sampling of the signal being transmitted to the receiver,already corrected for interference, is summed with the same localoscillator 27 output at junction 31 and is applied to another IFamplifier 29. Finally, both IF outputs are compared in the signalprocessor 32. To the extent that a residual interference signal remainsin the corrected signal from the sampling coupler 33, an additionalcorrection is generated by the signal processor 32, and applied to thesignal controller 24 to correct for the uncompensated error.

The signal processor 32 of FIG. 2 may be implemented in any of severalwell known ways to produce the amplitude (K) and time delay (T) outputsignals. One technique for generating these factors is to do so at atime when the desired signal is known to be absent. By definition, theonly remaining signal is the undesired one. At this time, the outputs ofboth ports could be applied to an amplitude comparator for producing anamplitude correction signal K, and simultaneously both signals could beapplied to a phase comparator generating a phase correction signal T.One time when the desired signal is normally absent in a radar is beforethe pulse transmission during which time reflected pulses are no longerbeing received. Additional circuits could be added to disable thetransmitter and enable the signal processor to any time under operatorcontrol.

A manual control may also be provided so that the operator could adjustK and T while observing the radar scanner, manually tuning for minimuminterference.

FIG. 3 is a simplified schematic diagram of one automatic signalprocessor embodyment. Both channels comprise a noise limiter 34 and 35so that the circuit will not react to random noise, but will react onlyto received signals large enough to overcome a predetermined threshold.Both channel signals are compared on the basis of amplitude and phase,the outputs being K and T correction signals of appropriate polarity andamplitude which, when applied through the signal controller, will resultin the complete cancellation of the interference signal. Sample and holdcircuits 38 and 39 are provided so that the K and T levels can bechanged only at times when the noise threshold has been exceeded by anactual interference signal. The input to the horizontal (H) channel ismultiplied by the constant K in amplifier 40 so that, for proper valuesof K, there will be an exact amplitude match at the amplitude comparator36. The resultant amplitude and phase factors are then coupled to thesignal controller and used as described above.

Although both FIGS. 1 and 2 cite the problem of a circularly polarizedjammer or interference source, the invention is not constrained to suchspecific situations only. If for example, the interference source ishorizontally polarized, it is readily seen from both FIGS. 1 and 2 andthe same operational principle discussed above, that the interference atthe receive line will be cancelled by the process described above. If,however, the jamming signal and the desired signal are verticallypolarized, the simplified schematic arrangement shown in FIG. 1 will notbe adequate for the cancellation of the jamming signal. One can remedysuch situation by changing the polarization of the desired signal. Sincea radar, provided with means of employing both linear and circularpolarizations, can always react to select the polarization most suitablefor its purpose in a jamming environment, a change of polarization of aradar signal usually does not pose any operational problems. For thecircularly polarized radar case, it is seen that one can establish aport in the receive antenna line where there will be a predominantinterference, in comparison with the desired signal. The signal fromthis port then could always be utilized to synthesize the appropriatecancellation interference as shown in FIG. 1.

From the foregoing, it may be seen that we have invented a system wherean interfering signal may be cancelled provided that there is adifference of polarization between interference and desired signals.Further, where the interference and desired signals are similarlypolarized, provision has been made to change the desired signalpolarization to create said polarization differences.

This discussion has used radar as an illustrative embodyment. However,it is clear that this system can be used to cancel interference in anyrf receiving system; including any kind of radio communication link.

The above described embodyments and methods are furnished asillustrations of the principle of this invention and are not intended todefine the only embodyments possible in accordance with our teachings.Rather, protection under the United Stated Patent Law shall be affordedto us not only to the specific embodyment alone, but to those fallingwithin the spirit and terms of the invention as defined in the followingclaims.

We claim:
 1. Apparatus for receiving a wanted radio frequency signal inthe presence of interference comprising: a first antenna polarized inthe same mode as the wanted radio frequency signal; a second antennapolarized orthoginally with respect to said first antenna; meanscoupling said first antenna substantially directly to a radio receiverfor receiving said wanted radio frequency signal; a signal controllercoupled to said second antenna for varying the amplitude and phase ofany signal received by said second antenna; means subtractivelycombining the output of said signal controller with the signals presentin said first antenna to radio receiver coupling means; means forsampling the signals present between said subtractively combining meansand said radio receiver; a signal processor coupled to said samplingmeans and to said signal controller to receive signal samples in thepolarization of said first and second antennas; said signal processorincluding means for comparing the phase of signals derived from saidfirst and second antennas, said means for comparing the phase of signalsderiving phase correction constants; said signal processor includingmeans for comparing the amplitude of signals derived from said first andsecond antennas, said means for comparing the amplitude of signalsderiving amplitude correction constants; said signal processor includingmeans for deriving the amplitude and phase correction constants for saidsignal controller from said comparing means; and means coupling theoutput of said signal processor to said signal controller forcontrolling the amount of phase and amplitude variation of said signalcontroller to produce a minimum interference signal at said samplingmeans.
 2. Apparatus for receiving a wanted radio frequency signal in thepresence of interference comprising; a first antenna polarized in thesame mode as the wanted radio frequency signal; a second antennapolarized orthoginally with respect to said first antenna; meanscoupling said first antenna substantially directly to a radio receiverfor receiving said wanted radio frequency signal; a signal controllercoupled to said second antenna for varying the amplitude and phase ofany signal received by said second antenna; means subtractivelycombining the output of said signal controller with the signals presentin said first antenna to radio receiver coupling means; first and secondsampling means, said first sampling means sampling first signals presentbetween said subtractively combining means and said radio receiver; alocal oscillator; a first mixer coupled to said local oscillator; meansintroducing the output of said first sampling means into said firstmixer whereby the signals sampled thereby are converted to anintermediate frequency; said second sampling means sampling signals fromsaid second antenna, a second mixer coupled to said local oscillator;said second sampling means coupled to said second mixer to convertsignals sampled from said second antenna to the same intermediatefrequency as the output of said first sampling means; a signal processorcoupled to said first and second mixers and to said signal controller toreceive signal samples in the polarization of said first and secondantennas; said signal processor including means for comparing the phaseof signals derived from said first and second antennas; said signalprocessor including means for comparing the amplitude of signals derivedfrom said first and second antennas at said intermediate frequency.